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MG MGB Technical - fuel guage voltage drop

hi,
as far as i know,the guy who had my mgb before me looks to of wired the temp guage in series with the auto fan,so no prob there.the prob i have is the fuel guage has a voltage drop,i have a good earth from the sender but the needle moves just on empty with a full tank.
ive tested the guage from the battery and it moves all the way,so would it be a problem if i give it a supply and bypassed the voltage stabalizer,
cheers
a davies

Hi,

Bypassing the regulator is not a good idea. Though the gauge would read, it would be high, ie when the gauge reads 1/4 you would run out of fuel. From your symptoms I would suspect a dodgy sender unit, float full of fuel or mechanically sticking.

Herb
Herb Adler

i have voltage at the live feed of the guage for a few seconds when the ignition is on then it pulsates is this right?
a davies

Check out Paul Hunt's The Pages of Bee and Vee. He has information of checking out the fuel sender and gauge in the technical articles. Yes, the voltage should pulsate as I remember the system working.

Les
Les Bengtson

Les remembers correctly. The voltage stabilizer works by pulsing the voltage on and off such that the average voltage fed to the gauges is 10 volts - difficult to see with a meter unless it is an analog meter that has a considerable amount of damping of the needle. Cheers - Dave
David DuBois

A Davies, I hope you mean the fan is in parallel and is from the fuse before the stabiliser! The stabiliser is does pulse as others have said. In fact it works just like a flasher unit only it should be "on" more. It's intended to provide a more stable 10v tether than the variable 12 to 14v you get from a battery and charging circuit. You can get modern electronic stabilisers on eBay for a few quid and I would start with this if you are unable to confirm the stabiliser is working properly.

MGmike
M McAndrew

Just connect the two wires at the tank sender together, and if the gauge then moves smartly towards F it is the sender that is at fault. Only do this for as long as it is needed to ascertain that the gauge does reach F.

You don't give the year of the car (always do this) but if you only have one wire and one spade terminal on the sender then short the wire to the body of the sender and the same thing should happen.

If that doesn't cause the gauge to rise to F, then there is a problem back towards the gauge. Earth the green/black bullet connector in the mass of connectors where the rear harness joins the main harness by the fusebox, both sides.

If still nothing then earth the green/black terminal on the back of the gauge. If one of these then shows 12v whereas the previous didn't then the fault lies between the two.

If still nothing check for 12v pulsing on and off about once per second on both terminals. If on the light-green/green but not on the green/black then the gauge is faulty (although you say it is OK on the battery).
PaulH Solihull

Since no one asked, let me tell you all that-- being a Bozo at the core-- after confirming proper sending unit function, volt stab function and proper gauge operation, I pulled the gauge out of the dash, popped out the little pad insulators and adjusted the bimetal strip, via the 2 slots exposed, so at 13 gallons in the tank the gauge needle just almost touches the "F". When it just brushes the red section down near "E", I have confirmed there are 4.5 gallons left in the tank at that reading. When the needle shows just to the right of the "E", fully in the red, I measured almost a gallon in the tank...hopefully enough to get to a pump. Oh, and I also carry a full one gallon plastic "can" jammed in the spare batt box hole, and a 2 pint bottle of "Spare Tank" ( a US product I can no longer find, kinda like gas, but not really, to get you to the off ramp or a gas station) which I've had unused for at least 7 years stashed in the corner of the trunk ( sorry, boot). I'd love Paul or Les to show me how to hook up a yellow LED to start blinking wildly when the needle gets below the red/black interface on the gauge. Shouldn't be too hard, eh? And yes of course we all know why I am fixated on this running out of gas thing, having hoofed too many miles, late at night it always seems, feeling like a clown. Cheers, Vic
vem myers

The flashing is easy - the factory 'stabiliser' will do that all on its own :o) All that remains is for a voltage comparator connected to the sender terminal of the gauge to prevent the LED turning on if the voltage is above a certain level, although still with 4.5 even US gallons seems a little previous to me. I've calibrated my gauge so that it reads E having run out and one gallon put back in.

The only time I have ever run out is just after changing the sender. I should have realised from the mileage (I zero my trip each time I fill the tank) but only started wondering when I noticed it wasn't dropping much below 1/4 tank. While it still went up a bit on left-hand curves I knew I was still OK, but when it stopped doing that I knew I was in trouble. I was coming up to a service area on the motorway and was driving as gently as I could, but then it expired. I knocked it out of gear and rolled a quarter mile or so, but unfortunately the last few hundred yards was uphill, and I even stopped about 10 yards short of a layby. After that I recalibrated the gauge, and carried a spare gallon for years, but of course never needed it.
PaulH Solihull

This thread was discussed between 28/10/2011 and 28/11/2011

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